1. Field of the Invention
This invention relates to a method for producing a sheet-formed recording medium, particularly to an improvement of its heat treatment process.
2. Description of the Related Art
A sheet-formed recording medium based on a base material comprising a flexible synthetic resin such as polyethylene terephthalate, polyimide, polyphenylene sulfide, or polypropylene, on the surface of which is formed a recording layer, when subjected to a high-temperature environment, tends to cause changes in dimensions due to an irreversible heat shrinkage. If this dimensional change occurs between recording and reproduction, a tracking error is caused, which impairs the reproduction performance.
Utilizing the fact that the dimensional change is irreversible, it has been proposed that a heat treatment step be added to the production process of a sheet-formed recording medium, in order to effect a controlled positive heat shrinkage.
Furthermore, in general, heat treatment not only removes the curling, warping, or internal stress of a recording medium, but also when a magnetic recording medium is heat treated, it enables enhanced running stability of a magnetic recording and reproduction head and improved strength and durability of the recording layer.
In this case, a problem occurs where the heat treatment is carried out in the production process. Specifically, when the base material in the rolled form is heat treated, heat shrinkage tends to be insufficient in the longitudinal direction, and unequal shrinkage occurs between the internal and external peripheries because internal pressure differs between the internal and external peripheries of the roll. To remove the problems with heat treatment in the rolled state, for example, Japanese Patent Publication Laid-open 59-127233 proposes a method in which a rolled base material is unrolled into a sheet which is cut, a plurality of the cut sheets are stacked and heat treated.
However, heat treatment of the cut sheet-formed base materials 11 which are stacked with their opposite major surfaces placed horizontally has posed the following problems:
(1) As shown in FIG. 18, when a foreign substance 12 such as a fine dust particle or a projection of the surface coating layer is interposed between the stacked sheet-formed base materials 11, the foreign substance 12 is pressed between the base materials 11 by the weight of the base materials 11 themselves, resulting in defects such as dents, deformation, flaws, or adherence. These defects are further emphasized by heat treatment, resulting in permanent defects which have major adverse influences on the recording medium.
(2) Among the stacked base materials 11, there is a difference in pressure applied by the weight of the base materials between the upper and lower base materials, which has a tendency to produce a difference in smoothness depending on the position in the stack.
(3) Since the stacked base materials 11 are in close contact with one another, heat gradually transmits from the outer periphery to the center of the stack. Therefore, it has been difficult to obtain uniform heating conditions over the entire stack of the base materials 11. Similarly, when the base materials 11 are humidified during heat treatment, a uniform quantity of moisture may not be able to reach the center of the stack of the base materials 11, resulting in an unevenness between the outer and inner portions.